As patterns are subjected to more or less rough usage, and are alternately wet and dry, it follows that the ideal material is one whose hardness is such that it will withstand the wear and tear of handling and at the same time be impervious to the effects of moisture. Such material is to be found in the metals, but, as the cost of working these into the proper shape is considerable, some kind of wood is usually substituted.
If, then, wood is to be used, another qualification is to be added - namely, it should be easily worked. The best wood for the purpose is undoubtedly white pine. Care should be exercised in the inspection of the wood, to see that it is clear, straight-grained, and free from knots. The straightness of the grain can be determined by the appearance of the sawed face which should present an even roughness over the whole surface.
The wood should be seasoned in the open air, but preferably sheltered by a roof, and should be piled so that the air has free access to all parts of the plank. In the natural process of air-drying, the moisture slowly works out to the surface and evaporates until the wood is dry or seasoned. One of the characteristics of wood is that moisture is readily given off from its surface if the surrounding atmosphere has a lower humidity, and also readily absorbs moisture in case of being subjected to a higher humidity. In kiln-drying, the stock is robbed of its moisture to a point below that normally contained in outside atmosphere. This means that every time some of the surface stock is removed, exposing a new surface, the stock at this surface will either attempt to absorb moisture and swell, or moisture will dry out, shrinking the stock, and in either case warping and disturbing the stock. This changing is always going on in pattern stock to some degree, but is less in stock that has dried or seasoned naturally to a point where there is about the same amount of moisture in the stock as there is in the atmosphere. It is best to keep the pattern stock for some time before its use as nearly as possible under the same atmospheric conditions as it is in while the pattern is being built. This holds good whether the stock is air-or kiln-seasoned.
It may be stated then, that, in the United States, white pine is the material commonly employed for pattern making. Lumber 1 inch, 1 1/4 inches, and 1 1/2 inches thick will be found convenient in the construction of such patterns as are most commonly called for. It results in a great saving of time and labor, after the lumber has been carefully selected, to have it taken to the planing mill and dressed on two sides to the following thicknesses: 1-inch, dressed on two sides to 7/8 inch; 1 1/4-inch, dressed on two sides to 1 1/8 inch; 1 1/2-inch, dressed on two sides to 1 3/8 inch; and, if such can be found well-seasoned, a small quantity of 2-inch, dressed to 1 3/4 inches. In addition to these sizes there should be a moderate amount of 1-inch resawed and dressed to 3/8 inch or to 5/16 inch; and the same amount of 1 1/4-inch resawed and dressed to 1/2 inch. The last two thicknesses are used for gluing and building up the rims of pulleys, gear wheels, and other light work, where strength and durability are required.
Although pine is in general the ideal wood for pattern work, it is soft and weak, so that, if small and strong patterns are desired, a harder wood is usually employed, Mahogany is much used for this purpose. Like pine, it is not liable to warp, and, when straight-grained, it is worked with comparative ease. There are many varieties of this beautiful wood, varying greatly in firmness of texture. The soft bay wood, often sold as genuine mahogany, should be avoided for patterns, being but little harder than pine. Cherry is also extensively used, but is not so easily worked to a smooth surface as mahogany, and is more liable than the latter to warp and to be affected by moisture. Black walnut, beech, and maple are used to some extent. Black walnut is stronger than cherry, but, like beech and maple, is likely to warp.
Observation shows that if one side of a board is kept damp and the other dried, the former will expand so that the plank, although originally straight, becomes curved, as in Fig. 1. Or if one side of a board is exposed to the air, while the other is more or less protected, as in the stack of boards shown in Fig. 2, the exposed side of the upper board will give off its moisture more rapidly than the other side, and the board will warp or bend in the direction shown by the dotted lines. The second board will also draw up and to some extent follow the first, being in turn followed by the third, and so on until the entire stack is warped and bent.
Fig. 1. Board Warped from Unequal Dryness.
The same thing will be found true of a well-seasoned board if after being planed it is allowed to lie on its side on the work bench. The upper side will give off its moisture more freely than is possible for the under side, the latter being protected and having its moisture retained by the bench. The lower side of the board is thus caused to expand, and the upper to contract, with the result that the board, although originally planed straight, becomes curved. For this reason all lumber, even if well-seasoned, should be so placed in racks, or on end, that the air may have free access to both sides of the planks; and newly planed boards, however dry and well-seasoned, should never be stacked together, but so placed that both sides will be exposed alike.
This tendency to warp is explained to some extent by the porous nature of all woods, and their inclination to give off or to absorb moisture according to the condition of the surrounding atmosphere. As there is always more or less moisture in the air, and lumber of ail kinds contains an amount of moisture which is ever changing according to the conditions of the surrounding atmosphere, this causes corresponding expansion or contraction of the wood.
Even under cover and in a dry place, wood has a tendency to warp on account of the greater shrinkage of the newer as compared with the older cells of the wood tissue or fiber in the side of the board nearest to the outside or sap wood of the tree. The inner side A of the board, Fig. 3, being closer to the heart wood, is older than the side B, and its cells are firmer and more compact than those of B. As the board seasons, the newer and more open cells of the side B shrink faster and to a greater extent than those of A, thus causing the board to draw or warp in the direction indicated by the dotted lines.
Fig. 3. Effect of Older Fibers in.
In gluing or building up stock for a pattern, this tendency may be corrected to some extent by reversing the grain of the pieces that are to he glued, and placing together two outsides, as B, or two insides, as A, Fig. 3. This is fully illustrated in Fig. 4.
In gluing very thin pieces together for the webs or centers of pulleys and for other purposes, it is often necessary to reverse the grain of the pieces, or to place the grain of one piece at right angles to that of the other, for the purpose of gaining greater strength and stiffness. In such cases, if only two thin pieces are used, the result, to some extent, after they have been glued and dried, is as shown in Fig. 5, the shrinkage and strain of the end grain crosswise of the board at a, being sufficient to bend the opposing thin board lengthwise of the grain at b, while on the side cd, the curve is reversed for . the same reason. Whenever it is necessary to cross the grain of thin pieces for a pattern, three or more pieces should be used, which will give satisfactory results if placed together, as shown in Fig. 6.
When thin circular disks of large size are to be glued up for patterns of any kind, the strongest, stiffest, and most satisfactory results wilt be obtained if the pieces are fitted and glued tangentially to the hub or other center or opening in the disk, as shown in Fig. 7. The grain of the wood must run lengthwise, and parallel to the longest side of each sector; and, after the pieces have been fitted together, a thin groove is cut in the edge of each, in which thin tongues of wood are inserted and glued, as illustrated in Fig. 8. Two disks are glued up, and one is turned over so as to reverse the grain of the sectors of one disk on that of the other, as shown by the dotted lines. The disks are then glued together, making a very rigid construction, and one which, owing to the continual change in the direction of the grain, will not warp.
Should a wide and thin piece of a single thickness be required for a pattern, the board from which it is to be made should be ripped into strips of 2-, 3-, or 4-inch width - according to the width of the required board - and the strips glued together again with each alternate strip reversed, as shown in Fig. 9. In this way warping is largely corrected, each narrow strip being inclined to warp in an opposite direction to that of its neighbor.
Fig. 6. Flatness Obtained by Crossing Grain of Three Thin Pieces.
Fig. 8. Interlocking Tongues.